CPESC Soil Science 3 — Questions and Answers
Question 1: What is the primary mechanism by which organic matter improves soil erodibility?
- Binding soil particles into stable aggregates (Correct answer)
- Increasing bulk density of the soil
- Reducing soil pH to improve plant growth
- Increasing surface runoff velocity
Correct answer: Binding soil particles into stable aggregates
Organic matter acts as a binding agent through compounds like glomalin and humus that cement soil particles into water-stable aggregates resistant to raindrop impact.
Question 2: In the context of soil erosion, what is 'rill erosion'?
- Erosion forming small, defined channels on a slope that can be removed by tillage (Correct answer)
- Sheet removal of soil in a thin uniform layer across a slope
- Deep channel formation that cannot be crossed by farm equipment
- Soil detachment by wind action along field margins
Correct answer: Erosion forming small, defined channels on a slope that can be removed by tillage
Rill erosion creates small defined channels caused by concentrated flow that are still shallow enough to be obliterated by normal tillage operations.
Question 3: A soil bulk density of 1.8 g/cm³ in a sandy loam would most likely indicate:
- Severe compaction restricting root growth (Correct answer)
- Optimal conditions for plant growth
- High organic matter content
- Well-structured aggregated soil
Correct answer: Severe compaction restricting root growth
Sandy loam soils with bulk densities above 1.75 g/cm³ are considered compacted, restricting root penetration and reducing infiltration rates.
Question 4: Which process describes the downslope movement of water-saturated soil material in a slow, plastic flow?
- Solifluction
- Creep
- Slump
- Earthflow (Correct answer)
Correct answer: Earthflow
An earthflow is the downslope movement of water-saturated fine-grained soil that moves in a plastic, viscous manner, often faster than creep but slower than a mudflow.
Question 5: In soil texture analysis, the hydrometer method is used to determine:
- The percentage of clay and silt by settling velocity differences (Correct answer)
- The aggregate stability of soil crumbs
- The organic matter content by oxidation
- The soil pH and cation exchange capacity
Correct answer: The percentage of clay and silt by settling velocity differences
The hydrometer method measures suspension density over time as particles settle by gravity, allowing calculation of clay and silt fractions based on Stokes' Law.
Question 6: Which soil order in the USDA Soil Taxonomy is characterized by a thick, dark, organic-rich surface horizon formed under prairie vegetation?
- Mollisols (Correct answer)
- Alfisols
- Ultisols
- Inceptisols
Correct answer: Mollisols
Mollisols are characterized by a thick, dark mollic epipedon rich in organic matter, typically formed under grassland or prairie vegetation in subhumid climates.
Question 7: What is the significance of the 'credible soil loss tolerance' (T value) in erosion control planning?
- Maximum annual soil loss rate at which soil productivity can be maintained indefinitely (Correct answer)
- Minimum slope gradient requiring erosion control measures
- Total sediment yield allowed to leave a construction site
- Target infiltration rate for revegetated disturbed areas
Correct answer: Maximum annual soil loss rate at which soil productivity can be maintained indefinitely
The T value represents the maximum rate of soil erosion that allows crop productivity to be sustained economically and indefinitely, typically ranging from 1 to 5 tons/acre/year.
CPESC Soil Science 3
What is the primary mechanism by which organic matter improves soil erodibility?